This document discusses microorganisms and diseases. It begins by defining disease and describing the main causes of diseases, including unhealthy activities, genetics, and microorganisms. It then describes the four main types of microorganisms that can cause disease: bacteria, viruses, fungi, and protozoa. For each type, it provides details on their structure, function, and examples of diseases they cause. The document also discusses the general causes and transmission of diseases, important infectious diseases, and the body's defense mechanisms against diseases like immunity and vaccination.

1. Microorganisms and Diseases
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Edexcel IGCSE - Human Biology - Chapter 08

2. What is a disease?
• A condition with a specific cause in which a part or all of
the body functions abnormally or less efficiently.
• Causes for diseases,
– Unhealthy activities – drinking alcohol, smoking
– Genetic – haemophilia, sickle cell anaemia
– Microorganisms – common cold, TB, pneumonia
• Infectious diseases – diseases that can be transmitted
from one person to another
• Pathogens – microorganisms that cause diseases
(bacteria, virus, fungi, protozoa)
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3. What is a disease?
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4. Types of microorganisms
1. Bacteria
– Very small single celled organisms
– 1 – 5 µm in length
– Basic shapes – spheres / rods / spirals
– Internal structure
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5. Types of microorganisms
– Structure of bacteria
• Surrounded by a cell wall made of polysaccharides and proteins
• some bacteria has a capsule of slime layer for extra protection
• It has no nucleus, instead it a genetic material in a single chromosome
• Some bacteria can swim using a tail called the flagellum
• Some have small circular rings of DNA, which carries bacterial genes
called plasmids
• Most bacteria harmless to humans
• Some carry out photosynthesis as they have chlorophyll in the
cytoplasm
– Importance of bacteria
• As decomposers – recycling waste in the soil, sewage treatment
• To produce diary products – lactobacillus bulgaricus is used in yogurt
production
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6. Types of microorganisms
2. Viruses
– They are parasites which can only reproduce inside a host (living
cell)
– Diameter → 0.01-0.1µm
– Not made of cells
– A simple particle which has no nucleus or cytoplasm
– It is composed of a core genetic material surrounded by a
protein coat
– Genetic material can be either DNA or RNA which requires to
reproduce inside a host cell
– Some viruses have an envelope stolen from host cell membrane
– They do not feed, respire, excrete, move, growth, respond to
their surroundings except reproduction
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8. Types of microorganisms
3. Fungi
– Most fungi do not cause diseases
– including mushroom, toadstools and moulds as well as
yeasts
– They are multicellular organisms
– Fed by secreting enzymes on to organic material,
breaking it down and absorbing the soluble products
– Important in decomposition and fermentation
– Diseases
• Athletes foot-skin disease caused by fungus feeding on
skin
• Farmer’s lung- disease caused by inhaling spores of a
mould
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9. Types of microorganisms
4. Protozoa
– Single-celled organisms
– Similar to a typical animal cell
– Mostly free-living, inhabiting, wet or moist areas
(ponds, rivers, seas )
– Diseases
• Dysentery caused by amoeba
• Sleeping sickness caused by trypanosome
• Malaria by plasmodium
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11. The general cause of a disease
• Incubation period
– the time period between when a person first shows signs and
symptoms of a disease
– The infected person may not feel sick but could be infectious to
others.
– Can vary from hours(eg: common cold) to years(eg: leprosy)
• Sign
– a disease visible to other people
• Seen – red patches
• Heard – abnormal lung sounds by auscultation
• Measured – increase blood pressure
• Symptoms
– What patient experiences from the disease
• Nausea, chill and pains
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12. Distribution of diseases
• Endermic-when a disease is always present in a
population of a particular geographical area
• Epidermic-widespread outbreak of an infectious
disease with many people becoming infected at the
same time
• Pandermic-when the disease is spread all over the
world(eg:swine flu)
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13. Important infectious diseases
• Viral – influenza, poliomyelitis, AIDS
• Bacterial- typhoid, TB, gonorrhea
• Fungal- Athletics foot, thrush
• Protozoa- Malaria,
• Parasitic worms- Schistosomiasis
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14. Viral diseases
1. Influenza
– Primarily infects cells in the upper air ways of respiratory system
– air born disease
– Symptoms
• Initially – cough, running nose, sore throat
• After about 5 days – fever and aches
• Others – loss of appetite, fatigue
– Can lead to pneumonia which can be fatal
– Viral influenza can lead to secondary infections like bacterial
diseases (bronchitis, bacterial pneumonia)
– The three strains of flu virus
• C – mild form
• A and B – mutate periodically and produce new strains which leads to
re-infection (genetic mutation of the virus replication)
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15. Viral diseases
– The treatment basically focused
on the symptoms.
– Antibiotics will be prescribed to
control secondary attacks usually
by bacteria.
– The spread of diseases can be
controlled by vaccination,
specially for extremities (babies
and elders) of age.
– People with flu must stay away
from other healthy people to
avoid spreading of infections.
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17. Viral diseases
2. Poliomyelitis
– Contaminated from human faeces to food.
– Motor neurons get destroyed
– 1% of polio patients get effected by infection in the
brain and spinal cord which leads to;
• muscle weakness and paralysis of the legs
• Can even lead to death because of respiratory paralysis
– It was appeared as an epidemic in Europe, UK, USA.
– Numbers reduced as the vaccination was introduced
and with good, safe sanitation.
– The goal of the WHO is to eradicate the disease.
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19. Viral diseases
3. AIDS (Acquired Immune Deficiency Syndrome)
– Caused by HIV (Human Immunodeficiency Virus)
– The virus infects T-lymphocytes
• Process;
↓ the genetic material is injected to the T-lymphocyte
↓ the activated HIV DNA instructs the lymphocyte to produce HIV proteins and genetic material
↓ Some of the HIV proteins and genetic material are assemble in the new viruses
↓ Some of the HIV proteins end up as marker proteins on the surface of the cell
↓ These HIV proteins are recognized as foreign proteins
↓ The lymphocyte is destroyed by the immune system
↓ The assembled virus particles escaped to infect other lymphocytes
↓ This cycle repeats itself for as long as the body can replace the lymphocyte that have being
destroyed
↓ The body will not be able to replace the lymphocyte at the same rate at which they have been
destroyed
↓ The free viruses in the blood increases rapidly and HIV may infect other areas of the body
↓ The immune system is damaged and other disease causing MOs infect the body
↓ Death can be caused as a result of infections by TB or pneumonia, due to the reduced capacity
of the immune system that would normally destroy the organisms. Death can also be caused by
rare cancers such as Kaposi's sarcoma.
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21. Viral diseases
– AIDS is primarily transmitted by unprotects sexual
intercourse or blood-to-blood contact
– AIDS can not be transmitted by kissing, sharing utensils,
giving blood, skin to skin contact or sitting on the same
toilet seat as the infected person
– Transmission of AIDS can be controlled by;
• Using of condoms
• Drug users not sharing needles, and using only sterile needles
• Limiting the number of sexual partners
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23. Bacterial diseases
1. Typhoid
– Is caused by Salmonella typhi.
– Is transmitted by contaminated food or water.
– It was endemic throughout the world and epidemic was common.
– Causes for transmission,
• Poor sanitation
• Natural disasters (flood)
– The incubation period is about 2 weeks
– Clinical features,
• Starts with a high fever, headache, cough, and general feeling of unwell.
• Then, stomach cramps, constipation, diarrhoea, vomiting, delirium.
• In severe cases, severe diarrhoea, gut bleeding, gut perforation and it may lead to
inflammation of heart and multi organ failure.
– Treatments,
• Antibiotics like penicillin are effective.
• Oral rehydration therapy.
– Prevention,
• Vaccination
• Good hygiene and sanitation
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25. Bacterial diseases
2. Tuberculosis (TB)
– Is caused by Mycobacterium tuberculosis.
– Is transmitted by droplet infection (airborne disease) when infected person cough, sneeze or
spits
– Common in developing countries rather than developed countries where over crowded and with
unhygienic living conditions.
– Usually infects respiratory system, damaging lung tissue, forming swellings called tubercles (can
be diagnosed by an X-ray).
– Can also infect other tissues like bones and lymphatic system.
– It can suppress the immune system which makes more vulnerable for the disease.
– Symptoms;
• Chronic cough (may with blood)
• Fever
• Night sweats
• Weight loss
– Treatment;
• Long term use of multiple antibiotics (usually takes 18 months to kill the bacterium).
• Resistant to treatment is a problem.
– Prevention;
• Vaccination – BCG
• Improve hygiene
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27. Bacterial diseases
3. Gonorrhea
– Is caused by Neisseria
gonorrheae.
– One of the sexually transmitted
diseases (STDs).
– Common throughout the world
– Symptoms;
• Male -
– Discharge of pus from penis.
– Pain when urinating.
– Fever and headaches.
• Females –
– Usually symptoms are not present.
– Discharge from vagina.
– Menstrual irregularities or vaginal
bleeding
– Once it is spread to uterus,
oviducts and ovaries can be
infertile.
– Treatment;
• Using combination of antibiotics
– Prevention;
• Avoid multiple partners or the
infected.
• Using condoms.
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29. Fungal diseases
1. Thrush
– Is caused by Candida
albicans.
– When a person is treated
long with antibiotics, it
may destroy the skin
bacteria which allows
fungus to grow out of
control.
– These are common with
people who has weak
immunity (babies).
– Treatment;
• Anti fungal drugs – local
application or oral drugs
2. Athletes’ foot
– Usually grows on warm,
moist skin between the
toes.
– Transmitted to the skin
from air or floor.
– Treatment;
• Anti fungal drugs – local
application or oral drugs
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30. Fungal diseases
1. Thrush 2. Athletes’ foot
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31. Protozoan diseases
• Malaria
– Is caused by plasmodium
– Insect host- female anopheles mosquito
– Refer the life cycle of malaria parasite
https://www.youtube.com/watch?v=WoIO-g1hiSo
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35. Parasitic diseases
• Schistosomiasis
– Is caused by a parasitic worm called schistosoma
– Usually not fatal but damages many internal organs and make them week
– Transmitted to the humans via fresh water snails so called as snail fever
– It often infects small children who swim in rivers where snails lives
– Most common in areas with poor sanitation and no access to clean drinking
water
– Symptoms
• Fever
• Chills
• Diarrhea
• Skin rash
• Blood in the urine
• Organ enlargement – liver, spleen, lymph nodes
– Life cycle of schistosoma; https://www.youtube.com/watch?v=8Btfge8AaoQ 35

36. Parasitic diseases
• Schistosomiasis
– Treatment and prevention;
• Drugs to kill the worms in the body
• Killing snails to interrupt the cycle
• Improve sanitation
• Health education
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37. Defence against diseases
• Immunity
1. General immune response
• Aiming at any invading MO
• Phagocytosis- some WBCs(phagocytes) ingest MOs forming pseudopodia.
2. Specific immune response
• Aiming specifically on different MOs
• Done by B and T Lymphocytes
• Each lymphocyte has slightly different receptor proteins that can bind with
antigens on MOs
• When B and T lymphocytes bind with an antigen it activates and rapidly
replicate into millions
• When B lymphocyte are activated they produce antibodies which binds with
antigens and cause MOs to clump together. so, can be easily killed by
phagocytes
• Some antibodies cause the cell to burst open.
• Some B lymphocytes do not kill MO but develop into memory cells and remain
for years
• Memory cells are capable of reproducing antibodies rapidly in a reinfection
from the same MO
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38. Defence against diseases
• Immunity
2. Specific immune response
• T lymphocytes do not destroy MOs in the same way.
• They are able to recognize infected cells or cancer cells from the
tell tale marker antigens on the surface of the infected cell.
• Then the destruction is done by two ways,
1. Some release chemicals that penetrate the infected cell to leak out the
contents.
2. Some activate the programmed cell death in their genetic code.
• Some T lymphocytes become memory cells and live for many
years.
– The above process is natural active responses made by our
own body to become immune for a disease called
immunization.
– Refer - https://www.youtube.com/watch?v=zQGOcOUBi6s
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39. Defence against diseases
• Natural passive immunity
– Acquiring immunity without producing an immune
response by ourselves.
– Receiving antibodies across the placenta during
pregnancy.
– Receiving antibodies via breast milk (collestrum –
rich in antibodies than proper breast milk ).
– Short lived immunity which is vital for the
newborns since the immunity is not developed in
them.
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40. Defence against diseases
• Artificial active immunity – vaccination
– The actual vaccine contains;
• an attenuated (weakened strain of the MO) – polio, TB, measles
• Dead microorganisms – whooping cough, typhoid fever
• Modified toxins of the bacteria – tetanus, diphtheria
• Just the antigens – influenza vaccine
• Harmless genetically engineered bacteria – hepatitis B
– Lymphocytes act on the above and builds up the specific immunity which long lasts for years.
– Travelers are advised to take vaccinations before visiting some countries.
– Vaccinations was introduced by Edward Jenner
– Refer - https://www.youtube.com/watch?v=jJwGNPRmyTI
– There’s a risk of vaccinating attenuated MOs when the receiver is immuno suppressed or the
MO is not properly weakened.
– Recent research suggested that there’s no link between MMR (mumps, measles and rubella)
vaccine and children getting autism
– Since some MOs mutate rapidly it is difficult to produce a unique vaccine (common cold).
– As flu virus mutates much slower than common cold virus it is possible to develop a vaccine.
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41. Defence against diseases
• Artificial passive immunity
– Vaccination is useless when people need protection against MOs immediately.
– So actual antibodies are injected passively to boost the own immune system.
• Rabies
• Louis Pasteur and Rabies
– Louis Pasteur has invented pasteurization
– He’s the one who developed a vaccination for rabies
– He had observed a way to reduce the virulence of the bacteria by culturing them
and was able to do that for anthrax and rabies too.
• Antibiotics – the discovery of penicillin
– Alexander Flemming discovered penicillin along with Howard Florey and Ernest
Chain.
– Refer - https://www.youtube.com/watch?v=0t_hrtju4eA
– Over use of penicillin developed a resistance through out the world.
– So many other antibiotics were discovered(chloramphenicol, aureomycine)
– Doctors use to prescribe combination of antibiotics for a greater effect but the
resistance was developed in bacteria for several antibiotics too.
– This lead to chose antibiotics sensibly.
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42. Defence against diseases
• How antibiotics act
– Bactericidal antibiotics - kills bacteria.
– Bacteriostatic – stop the reproduction of bacteria.
– Broad spectrum antibiotics – act against range of bacteria.
– Narrow spectrum antibiotics – act against few types of bacteria.
Example Mode of action Effect Type of antibiotic
Penicillin Stop the manufacture of
the bacterial cell wall
Water enters through
the weakened cell wall
and burst
(osmoticclysis)
Bactericidal
Nalidixic acid Interferes with DNA
replication
Stops the cell division or
multiplication
Bacteriostatic
Tetracycline Interferes with protein
synthesis
No enzymes to control
the bacteria
Bactericidal
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43. Defence against diseases
• Why antibiotics have no effect on viruses?
– Viruses don’t have cell walls
– Viral DNA only replicate inside other cells
– Viruses synthesis their proteins through the cell
– Antibiotics can’t enter human cells
• Producing penicillin
– Penicillin is secreted by the fungus penicillium.
– Any strain that produce a new type of penicillin has a different
antibacterial activity.
– Biochemist design various penicillin types altering the basic penicillin
molecule, using two techniques;
1. Modifying the side chain of the penicillin molecule by altering the diet of the fungus.
2. Adding artificial manufactured side chains to the penicillin with no side chains.
– These techniques allows different semi synthetic penicillin types which
have different antibiotic activity.
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44. Defence against diseases
• Food hygiene
– Preventing transmission of foodborne MOs can be done
at various stages;
1. Prevent MOs getting into the food.
2. Terminate the multiplication of MOs on the food (food
preservation).
3. Cook food properly to kill MOs.
– Avoid transmission of MOs to the foods at home;
• Cooked and raw food should not be stored together.
• Foods that have been frozen should not be refrozen after
cooking.
• Food not should be left open to the air.
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45. Technique Principles and procedures Example Drawbacks
Salting High salt concentration prevent
bacteria from multiplying and are
killed by losing water – osmosis.
Meat, fish, vegetables Some bacteria can withstand very
high salt concentrations.
Pickling Foods are bottled in vinegar which
lowers the pH and inactivates most
MOs.
Onions, red cabbage, herrings,
mayonnaise
Alters the taste.
Pasteurization The food is heated to 63 and 65°c for
30 mins which kills the pathogenic
bacteria.
Milk, cream, ice cream, fruit juice,
beer
It does not kill all bacteria, spores of
harmful bacteria may survive.
Ultra-heat-treatment (UHT) Superheated steam at 135 - 160°c is
blown through the food for 2 seconds
which kills all the bacteria and spores.
Milk Alters the taste.
Canning Food is packed in cans, heated to high
temperatures, seal and then
reheated to 105 - 160°c. This kills
bacteria and spores.
Beans, soup, tomatoes Cans can be damaged and this allows
bacteria to enter the can and
multiply.
Drying Drying removes water from the food
so bacteria cannot digest and absorb
it.
Cereals, grains, some fruits Water can easily renter once the
packets are open, bacteria can cause
spoilage.
Freezing Foods are cooled rapidly to -10°c, this
prevents the formation of large ice
crystals which could alter texture and
the flavor of food.
Many vegetables, prepared meat None.
Irradiation High energy gamma radiation is
passed through the food which kills
all bacteria and spores.
Some countries permit irradiation of
potatoes, onions, shellfish, some
fruits.
Food maybe made radioactive or
carcinogens maybe formed.
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